The present invention relates to a degreasing composition for treating a metal surface. More particularly, the composition of the present invention has a superior degreasing activity with extremely low foaming ability and, moreover, its degreasing activity is hardly deteriorated by a defoaming agent which is optionally added to the composition or fats and oils which adhere to the surface of the object to be treated and are carried into a degreasing bath or tank where the composition is used.
In the field of metal finishing, particularly, chemical conversion treatment of metal surfaces, there have been used various degreasing compositions for removing fats and oils (e.g mineral oils, animal oils, vegetable oils etc.) which adhere to the surface of the metal to be treated before the chemical conversion treatment. Generally, as such compositions, aqueous solutions containing as main ingredients a builder mainly composed of an acid or an alkali, and nonionic and/or anionic surfactants are used. These compositions can be used for treating metal surfaces with or without being diluted with water or the like. When a spray type metal surface treating equipment is used for degreasing or cleaning metal surfaces in metal finishing, foaming ability of a degreasing composition should be as low as possible.
There have been proposed various degreasing compositions having a low foaming ability.
For example, a certain surfactant which has a low foaming ability due to its specific structural formula is added to a degreasing composition. Examples of such surfactant are those described in Japanese Patent Publication No. 4373/1961 which are represented by the general formula: ##STR2## wherein R is an alkyl group; EO is ethylene oxide group; PO is propylene oxide group; and nl and ml are the number of moles of addition, respectively.
However, as the molar amount of the lipophilic group such as the propylene oxide group increases, not only does the foaming ability of the surfactant of the above structure decrease, providing a surfactant of sufficiently low foaming ability, but also its degreasing activity is lowered.
Further, degreasing compositions having a low foaming ability have usually been prepared by combining a hydrophilic surfactant which has strong degreasing activity and high foaming ability, and a lipophilic surfactant which has weak degreasing activity and low foaming ability. For example, a combination of a dialkylphenol alkylene oxide adduct and a higher fatty acid salt is disclosed in Japanese Patent Publication No. 4663/1968. A combination of an alkylphenol ethylene oxide adduct and a polypropylene glycol ethylene oxide adduct is also proposed.
However, in the above compositions, it is difficult to add a larger amount of the lipophilic surfactant having a low foaming ability, so as to lower the foaming ability of the composition, without deterioration of degreasing activity thereof.
Recently, for the purpose of preventing environmental pollution, a closed type metal surface treating equipment has been employed in a degreasing or cleaning process of metal finishing. When the degreasing or cleaning procedure is carried out in a closed type equipment, fats and oils which adhere to the surface of the metal to be treated can be collected in a degreasing bath or tank, where a degreasing composition is used, within a single system. The fats and oils thus collected in the tank can be removed with a suitable oil-water separator and then disposed of separately, by which environmental pollution can be prevented.
In a closed type metal surface treating equipment, it is also desirable to use a degreasing composition having a low foaming ability as described above. However, when a surfactant, particularly, a nonionic surfactant, is added to the degreasing composition, all or a part of the fats and oils in a degreasing tank are emulsified, and they cause deterioration of degreasing activity of the composition. There have been various attempts to restore the degreasing activity thus deteriorated but sufficient results have never been obtained.
For example, in one attempt, a concentration of a surfactant in a degreasing tank is made higher in order to restore the degreasing activity of the composition. However, this attempt results in significant foaming in the tank. Moreover, when a concentration of emulsified fats and oils (hereinafter, referred to as "emulsion-oil") in the tank becomes more than 5,000 ppm, it is difficult to sufficiently restore the degreasing activity and, eventually, the degreasing composition in the tank should be scrapped.
In another attempt, the degreasing activity is restored by destroying the emulsion-oil and then removing fats and oils. For example, it is described in Japanese Patent Publication (not examined) No. 91828/1976 to use an ultrafiltration equipment for this purpose. To use a centrifugal equipment is also disclosed in Japanese Patent Publication (not examined) No. 90934/1976. However, these attempts require a large cost for equipments and for complicated maintenance of the equipments.
Further, there is described another attempt in Japanese Patent Publication (not examined) No. 77269/1975. In this attempt, degreasing activity is restored by destroying emulsion-oil with an acid and then, after removing fats and oils, neutralizing with an alkali. However, this attempt also requires a large cost of the acid and alkali and further requires a filtration step which causes lowering of efficiency of the procedure.